1. Field of the Invention
This invention relates to a method and apparatus for joining a subsea pipeline to a riser attached to an offshore petroleum recovery platform. The apparatus is particularly useful for joining large diameter pipes at seabed depths where the effectiveness of divers is severely limited.
2. Description of the Prior Art
U.S. Pat. No. 3,599,436 describes a method and apparatus for joining a subsea pipeline to a riser wherein a submersible work chamber is placed over the adjacent ends of the submerged pipeline and the riser. Sea water is evacuated from the chamber under high pressure and the pressure is maintained to permit divers to weld the ends together. This method and apparatus is obviously limited to uses at depths at which divers can work with reasonable efficiency. The limiting depths are generally acknowledged to be about 250 to 300 feet.
Another method and apparatus for joining a subsea pipeline to a riser is exemplified by that shown in U.S. Pat. No. 3,707,077. Here a portion of the pipeline initially laid on the seabed is raised to the surface and joined to a riser bend section. The pipeline is then lowered and simultaneously additional sections of the riser are added to the bend section as the completed riser is installed on the platform. This technique is also limited to relatively calm shallow water, approximating 150 feet in depth, by the pipe handling capability of surface derrick and davit equipment.